Thin film interference filters are one type of flat optical component widely used in systems for optical measurement and analysis, such as Raman spectroscopy and fluorescence imaging, for example. Thin film interference filters, including optical edge and band pass filters, notch filters, and laser line filters (LLFs) are advantageously used in such systems to transmit light having specific wavelength bands and to reflect other light, including light that could otherwise constitute or generate spurious optical signals and swamp the signals to be detected and analyzed. Dichroic beam splitters utilize interference filter effects to reflect certain wavelengths or ranges of wavelengths and transmit other wavelengths or ranges of wavelengths.
Failure or poor performance of such filters compromises the performance of systems in which they are used. Considerable design effort and expertise are required to fabricate thin film interference filters of high quality. Conventional design approaches for optical instruments that utilize thin-film filters are often constrained by inherent characteristics of these filters and long-standing practices for how these filters are designed and used.
A particular concern relates to maintaining the optical surface so that it lies as flat as possible. The surface flatness of dichroic beam splitters, for example, affects a number of factors in the performance of a fluorescence microscopy system. For light incident at high angles of incidence, such as the 45 degree angle of incidence (AOI) typically used for a dichroic surface, the beam axis for transmitted light can be slightly laterally shifted relative to the axis of incoming light to the surface. Furthermore, if both opposite surfaces of the dichroic are appreciably curved, such that the dichroic filter has the shape of a bent parallel plate, the beam axis for transmitted light can be slightly diverted and therefore non-parallel to the axis of incoming light to the surface.
Light reflected from a tilted surface presents even more of a problem for dichroic filters as well as for precision reflective surfaces in general. For example, unless the reflective surface is flat to within close tolerances, the focus of an excitation beam from a light source can be shifted along the axis away from the focal plane of the focusing lens and the size of the focused point can be compromised. Similarly, the focal plane of the emitted light that is reflected by a dichroic beam splitter can be shifted along the axis of light away from its intended detector and the acquired image can be distorted.
There can be additional problems related to filter flatness with specific types of microscopy systems as well. For example, for a type of laser based fluorescence microscopy termed Total Internal Reflection Fluorescence or TIRF microscopy and for Structured Illumination microscopy, the relative flatness of the dichroic surface affects how well the measurement apparatus performs. If there is unwanted curvature of a dichroic beam splitter, the position of the focal plane can shift perceptibly and the size or shape of the focused spot change. Either of these two effects, or their combined effect, can significantly compromise the image quality obtained. Aberrations resulting from this focal shift and degradation may not be easy to correct and can adversely affect the overall imaging performance of the microscopy system.
Imperfect flatness can be a particular problem when using dichroic beam splitters with laser light, whether in microscopy or in other applications. For this reason, dichroic surfaces rated for use with lasers must meet higher standards for flatness and are more costly than dichroic surfaces that are used for other light sources.
Dichroic coatings are typically formed by thin film deposition techniques such as ion beam sputtering. These fabrication methods require deposition onto a flat substrate, but tend to add significant amounts of mechanical stress as they are applied. This stress, if not corrected in some way, can cause some amount of bending or warping of the underlying substrate, frustrating attempts to maintain suitable flatness. For this reason, many types of commercially available dichroic beam splitters encase the dichroic coating within a prism. Encasement solutions, however, present other problems, including practical difficulties in alignment, polarization splitting of spectral edges, the need for optical adhesives that can both withstand the optical and temperature environment while closely approximating the refractive index of the surrounding glass, and other shortcomings. The dichroic coating works best when it is disposed directly in the path of incident light; encasing the coating within glass or other substrate introduces optical problems, such as absorption and scattering, that can degrade optical performance.
Thus, there would be advantages to methods that would allow surface flatness for dichroic and other optical surfaces to be maintained within tight tolerances.